Interest in single-site (metallocene and non-metallocene) catalysts continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of .alpha.-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes include cyclopentadienyl, indenyl, or fluorenyl groups, which may contain other substituents or bridging groups. More recently, single-site catalysts in which a heteroatomic ring ligand replaces a cyclopentadienyl group have appeared, including boraaryl (see, e.g., U.S. Pat. No. 5,554,775), pyrrolyl (U.S. Pat. No. 5,539,124), and azaborolinyl groups (U.S. Pat. No. 5,902,866).
Many single-site catalysts require high levels of an alumoxane activator (e.g., polymethalumoxane). When used at such high concentrations, alumoxanes cause chain-transfer reactions that undesirably limit polyolefin molecular weight. In addition, high residual aluminum in the polymer adversely impacts mechanical properties, so the polyolefin product is normally treated after manufacture to remove it.
Boron compounds such as triphenylcarbenium tetrakis-(pentafluorophenyl)borate or tris(pentafluorophenyl)borane can be used instead of alumoxanes to activate some single-site catalysts. Unfortunately, however, these catalyst systems are usually less active and less stable than alumoxane-activated catalysts. Because boron-containing activators eliminate the need for high levels of alumoxanes, it would be valuable to develop a process that retains this advantage, yet overcomes the activity and stability issues.
Single-site catalysts also generally lack thermal stability compared with Ziegler-Natta catalysts. Olefin polymerizations catalyzed by single-site catalysts are normally performed at relatively low reaction temperatures (less than about 100.degree. C.) to prolong catalyst lifetime. Higher reaction temperatures would ordinarily be desirable, however, because polymerization rates generally escalate with increasing temperature. Thus, a process that enhances the thermal stability of single-site catalysts and allows higher reaction temperatures to be used would be valuable.
Prepolymerization of a small proportion of olefin with single-site catalysts and alumoxane activators to make prepolymer complexes is known. For example, U.S. Pat. No. 4,923,833 teaches to prepolymerize a portion of ethylene with bis(cyclopentadienyl)zirconium dichloride and an activating amount of methalumoxane, followed by addition of the prepolymer complex to a second reactor that is used for the main polymerization. U.S. Pat. No. 5,308,811 similarly teaches to use a prepolymerization technique with an alumoxane-activated single-site catalyst (see column 11 and Examples 23-26). Neither reference explains why prepolymerization is used, and neither suggests making the prepolymer complex substantially in the absence of an alumoxane or in the presence of a boron-containing compound.
An improved process for making polyolefins with single-site catalysts is needed. In particular, a process that gives products with low residual aluminum is required. A preferred process would avoid chain-transfer reactions and allow the production of high-molecular-weight polymers. A valuable process would use a boron-activated catalyst with both high activity and good stability. Ideally, the polymerization process could be performed at relatively high reaction temperatures without significantly deactivating the catalyst.